1. Field of the Invention
This invention relates to the field of aggregate and laminate compositions. Specifically, it relates to a method for bonding lignocellulose-containing raw materials with a urethane-modified diphenylmethane diisocyanate mixture.
2. Description of the Prior Art
The preparation of panels or molded parts of lignocellulose-containing fibers, chips or layers using isocyanate group-containing bonding agents, especially those containing di- and polyisocyanates is known (German Pat. Nos. 2,109,686 and 2,711,958).
Among other properties, such bonding agents are intended to improve the materials with respect to their stability and their behavior with respect to moisture.
It has been found, however, that a relatively rapid reaction of the isocyanate groups with water takes place when, for example, wood chips are bonded with known di- and polyisocyanates and/or their aqueous emulsions. This reaction is caused on the one hand by the undesirable but always present residual wood moisture and on the other hand by the use of aqueous emulsions. The effect is particularly pronounced if aqueous bonding resins based on amino and phenol plastic resins are simultaneously used. Due to their alkaline properties, they increase the reaction rate of the isocyanate groups. This results in a very narrow processing latitude as well as in frequently occurring scattering in the stability values of the materials bonded in this manner. Furthermore, in case of plant shutdowns or delays, solids can separate in the bonding agent. This results in blocking of the metering and measuring devices and plugging in the lines.
The application of 2,4'-diphenylmethane diisocyanate has been suggested for various purposes, for example, for manufacture of electrical insulation, coatings or floor coverings (German Published Applications Nos. 19 23 214, 24 47 625 and 26 23 346).
In the context of bonding agents for lignocellulose-containing products, however, there is a teaching that an amount of 2,4'-diphenylmethane diisocyanate in excess of 8 percent by weight brings about unsatisfactory results (German Pat. No. 2,711,958)
These drawbacks can be avoided if the bonding agent used is a prepolymer based on a diphenylmethane diisocyanate mixture.